Abstract

The flow regime of Tooms River, a tributary of the unregulated Macquarie
River in eastern Tasmania, was modified in 1840 by a shallow dam in its
upper reaches, resulting in reversed seasonality of high and low flows: a
classic, long-term example of a river subjected to “anti-drought”, but
without the confounding thermal effects of impoundment stratification. I
found surprisingly large differences between sites on this regulated
branch and sites on the adjacent unregulated branch of the Macquarie,
which persisted for 8 km downstream despite tributary inflows. This 2-
year comparative study coincided with a prolonged supra-seasonal
drought, which enabled examination of the effects of reduced stream flow
on both regulated and unregulated examples of confined rivers in a
naturally variable climatic regime. The regulated sites remained distinct
from the unregulated sites, in terms of riparian litter fall, biofilm
abundance and macro-invertebrate diversity.
This is one of very few studies of the long-term effects of flow regulation
(e.g. see Kondolf and Batalla 2005; Sheldon and Thoms 2006b), and
differs from the majority of regulated river studies in that the effects of
regulation of Tooms River are not confounded by land use practises or
thermal stratification of the reservoir. However, this impoundment is
typical of small irrigation impoundments in Mediterranean and semi-arid
climate zones. The ecological consequences of drought and “antidrought”
in a region with a naturally variable flow regime may inform
management of biodiversity in other regulated rivers in similar climate
zones. Human population increases and climate change pressure on
water supplies are likely to increase the demand for many more dams of
this size and type of flow alteration (Benstead et al. 1999). The
determination of the long-term effects of flow regulation is essential, so
that future decisions on water allocations can be based on better
knowledge of the impact on downstream ecosystem services.Prolonged regulation of the flow regime of Tooms River has had major
effects. Regulation has not only reversed the seasonality of flow, but has
also dramatically reduced flow variability, resulting in contraction of the
stream channel immediately downstream of the dam, and the
establishment of mature eucalypts close to the channel, in contrast to
the shrubby riparian vegetation maintained by the variable flow regime
of the Macquarie River. The timing of allochthonous litter inputs to the
benthos was altered, with peak litterfall delayed until irrigation demand
fell at the end of summer. In addition, there was reduced lateral
connectivity, with movement of leaf litter between the riparian zone and
the benthos dependent on overland flow. As predicted by other studies,
biofilms were more abundant in the regulated river, but the closed
canopy immediately downstream of the dam did not suppress
autochthonous productivity, against expectations. Chlorophyll a
analyses indicated an autotrophic system, dominated by cyanobacteria
and diatoms, in contrast to filamentous algae and diatoms in the
unregulated river.
Macroinvertebrate communities were significantly different between
rivers, but also between sites within rivers, with a depauperate fauna
closest to the dam. There was partial recovery of regulation effects at the
downstream regulated site, but the invertebrate fauna remained distinct
to the unregulated river. In contrast to the majority of studies, there was
little difference in invertebrate species richness or evenness between
sites or rivers, but diversity was marginally higher at the unregulated
sites. Invertebrates from the adjacent Macquarie River were expected to
colonise lentic habitats in the regulated river during the drought, but
this did not eventuate, suggesting that there may be significant barriers
to dispersal or colonisation.
Stable isotope analyses indicated a clear separation of the invertebrate
food webs of the two rivers. Although both rivers had abundant leaf
litter, terrestrial resources were a minor dietary component in both rivers. Vascular macrophytes dominated unregulated river diets, while
cyanobacteria were more important in the regulated river. Feeding
preference trials for the dominant terrestrial leaf species and for
macrophytes showed few preferences for terrestrial leaves, despite large
differences in toughness, C: N ratios and tannin content. Unlike other
Australian and overseas studies, most invertebrates showed a clear
preference for the macrophyte Triglochin procerum. This was supported
by the stable isotope analyses and suggests that the prevalence of
macrophytes in invertebrate diets may be underestimated in many food
web studies.
It is likely that similar long-term effects will eventuate in other regulated
rivers in Mediterranean climate zones as irrigation dams mature.